Welding fittings onto steel hull

Hi. Please forgive what might be a dumb question and any lack of forum etiquette from my ignorance/inexperience.

I would like to weld a number of steel rings (fixed on 5mm plate) to a narrowboat hull (6mm thick). It will be above the waterline (freshwater and brackish) but liable to regular splashing.

The plates are 45mm high and 25mm wide. I did a basic stick welding course years ago and remember being told that welding both sides of a fillet joint (is this a wide fillet joint?) can cause one side to crack as the other contracts and cools.

Should I weld all the way around the plates to stop water getting behind, or just down one side? I noticed we have some very experienced welders on this forum and any advice is much appreciated.

 

Ps. I will stick weld (no Mig welder yet)

 

If you are welding flat plates on to a flat surface, you will want to weld continuous so no water gets between as the rust will bleed out forever.
You do not have to worry about welding both sides of a fillet on mild steel and creating enough stress from contraction to crack the welds.
Cast iron, high alloy aluminum, I have seen this happen but should not happen with your installation
The trick would be to weld the perimeter up to 90%, allow it to cool, then do a final inch or so. This will allow the air between the plates to get to ambient temperature and not create a lower than atmospheric pressure between the plates
which could suck up water if the joint is not 100%
This is picky, but does not cost anything to do it this way

 

Hi Someofthegear,
Definitely seal weld around the whole perimeter.

Jeff.

 

Thank you both, much appreciated!

 

Make sure you can see the inside surface of the hull and that there are no cables or flammable materials. Some paints can also burn quite freely and start a fire. In the yard we always have a fire watcher with a co2 extinguisher inside.
If it's a while since you have welded your welds are likely to be a bit porous in any case. Clean up the weld and burnt surfaces (inside too) as soon as they have cooled and coat generously with a decent epoxy such as Jotamastic 87 or Hempel Hempadur these will seal the porosity and effectively prevent corrosion for many years.

 

Thanks Nick. Fortunately they line up with a slanted interior panel under the side deck, so i should have at least a foot clear once i've removed the old wiring. I will have a mate with an extinguisher on hand and keep an eye on them for a few hours afterwards.
I'm a big fan of Jotun paints and will be epoxying the outer hull - will remember to clean and coat the back of the welds too - hadn't thought of that.
Will also do plenty of practice runs first. I'll say 'out of practice' rather than 'rusty' (hopefully!).
Thanks again for all the advice and I will have plenty more questions I'm sure. This is a great site!

 

I'm not sure if i should start a new post for this or resurrect an old one? Apologies if I got it wrong.

It turns out welding fittings on was a minor detail of the work required

The boat (55' narrowboat) was slipped and I began the horrendously slow process of taking it back to bare steel (rotating blaster disks). The surveyor found the original plates (5mm) to be good overall but have extensive pitting in some areas, i.e. Too much to weld fill.

He has recommended over plating 4-5 sections approx 8x2' each and mainly flat. I would much prefer to replate, but apparently in these circumstances and on this boat, overplating is more economical. The budget is blown at this stage so I'm doing everything I can myself.

Some of the areas were back to bare metal, others i'm in the process of wirebrushing the worst off and using vactan across all to hold off the rust while I source steel.

I've got hold of MIG welder and reasonably confident of welding watertight joints onto the existing plate with enough practice runs. My vertical stick just isn't good enough.

If I overplate, I want it to be as good as possible in the limited time I have on the slip. What should I coat the outside of the original plates with (before welding on new) to preserve them as much as possible? Epoxy over the Vactan?

The new plate should be blasted and shop primed, so hoping I can Jotamastic straight onto the outside of the new plates.

Everything I've read and my gut feel says replate, but if the value of the boat, time available, my skills and budget mean I have to overplate, then I'd like to try and do the best job I can.

As always, any help and advice from you all is much appreciated. Hopefully all this will be good practice for bigger and better things, but a little daunted at the moment and the days are ticking by.

 

I'm presuming this is under the water pitting on the outside, probably around the waterline? How thin is the hull in the thinnest areas? Pitting doesn't look nice but it would have to be very severe to be a risk to a canal boat (if the corrosion was stopped), If you haven't already, I would get a second opinion from a boat builder before buying steel. Is there an insurance issue or other that is forcing you the make repairs? Is the hull corroded inside too or clean?
Over plating isn't allowed where I work so I haven't much experience of it except in very temporary repairs but if I was doing it I don't think I would spend much time preparing the steel between the plates though I would clean the weld areas carefully and also remove any paint between the plates that is going to get hot from welding as it will create gas which will blow out through the weld and give lot of porosity problems. For the same reason I wouldn't paint between the plates and if the weld is good it shouldn't be necessary in any case since it will be sealed.
Jotamastic should be fine on the new plates. Do you have access to an airless sprayer? It gives a much better coat when sprayed, it is difficult to roll or brush it without thin spots. If you have to put it on by hand then plan for a few coats and get different colours for each coat. Brushing or rolling in alternate directions for each coat can help to avoid thin areas.

 

Hi Nick, thanks for the quick reply. Yes, pitting is below or on waterline, sporadic from just above the baseplate up to the first rubbing strake. They range from shallow to 2.9mm. Plenty of 'high twos' so the steel is down to just over 2mm in places. One was below two.

There's no insurance request as yet, but if I sell to upgrade to a widebeam or barge, I'll need a clean hull survey to make the most of selling on the mooring. If I keep the boat for a few years, I want to limit deterioration before selling (6'10" is just too narrow!)

Inside, where I can see looks fine. There is some light rust on the top of the baseplate which I'll brush out when I reballast and get some paint or barge grease down there. The sides are pretty inaccessible until I start removing the lining to weld.

I was planning to clean the plate edges back 2-3 inches with a flap disk on both original and new plates. Should I worry about the holding primer on the inside of the new? I'm not sure how far back it would burn off?

It's a 'waterbased holding primer suitable for over painting with most paints'. I wasn't sure if the Jotamastic would go over it as well as bare metal, but I can't face using a Tercoo on brand new plate!!

I don't have an airless sprayer, but I have grey, redtone and black for three coats by hand. Hopefully the thin spots won't all be the same place

 

I presume you mean the holding primer that the new plate comes with? It should be weldable but it will still make gas which will vent through the weld. The primer will clean off the new plate very easily with a 40grt flap disk. Cleaning back about 15mm from the edge on the inside face should be sufficient, no need to go much more.
There shouldn't be any problems to overcoat it with Jotamastic. If the plate has been handled a lot then a quick wipe down with No 17 thinner or acetone may be a good idea.
Incidentally, vinegar followed by soap is excellent for removing jotamastic from skin.

I haven't used the Tercoo discs. What are your impressions? Do they clean to the bottom of the pitting or skip over the deepest areas? How long do the disks last? Fast or slow?

When you come to fit your new plate you'll probably have some difficulty to make it lie flat against the existing. The traditional way would be to tack dogs to the hull and wedge the plate in but you can do the same with a length of flat bar on edge used like a lever with a corner lightly tacked to the hull just by the new plate, there is less grinding after but you have to have one hand for the lever and one for welding. Another way is to tack the plate as best you can with a small tack and then while the weld is hot (works best with two people) give the tack a smart whack with a hammer to close the gap.

 

I am assuming that you are getting the sheet primed at the mill. ie holding primer
Have you supplier specify the thickness of the coating and check the manufacturers spec as to the minimum dry thickness. Often steel that is shop primed at the mill is an extremely thin coating so you can weld through it but frequently does
not meet the paint manufacturers requirements for minimum thickness.

You should also ask what the treatment of the steel was prior to the application of the shop primer, shot blasted, degrease and primed, wheel a braded, degreased and primed, or sand blasted, degreased and primed.
As the steel will most likely be hot rolled sheet, the scale must be removed entirely for a primer to have a chance of doing its job.

If you are doing the priming or spot priming after, a slower drying primer is better as it will allow better adhesion to the substrate.

 

Thanks for the tip re vinegar, the way I paint, that will come in very useful

Re the Tercoo, I'm actually using the Perago versions. They look identical (blue rather than brown) but I could get a seven disk multi version that would fit a M14 polisher spindle.

They are good but very slow. They take off the bitumen very quickly but the shop primer below takes a while. Any rust below that takes longer. You have to be pretty gentle or the teeth rip out, especially over any raised profiles.

They will clean to the bottom of the pits providing they are wide enough to allow the teeth to strike as part of their arc. I didn't notice much speed increase with the multi disk over the triple mounted in a drill, as the polisher is heavier and less nimble.

I'm running the multi at about 3,500 rpm and the triple at probably 2,500.

They last ok as long as you are very gentle and stick to flat areas. I would recommend for smaller areas or when you have a lot of time. If i did this hull again, I would wait until I had the money to get it grit blasted.

I was going to use dogs off the rubbing strakes, but will try the lever and hammer methods and see hiw i get on. For the lever, do you it will pivot on the tack weld?

Thanks again for all the help. Tomorrow I'm taking a circular saw to the lining, so the biggest challenge will be to keep cheerful!

 

Hi Barry. I sent the datasheet for the holding primer to the paint supplier and they said Jotamastic will be fine directly over it. It's sprayed on as a very thin coat and dried by the same machine after shot blasting apparently.

 

It's not difficult to rent the gear and do blasting yourself and much less expensive though it still costs a lot. I started blasting a few years ago to complete a steel boat project but there was a demand for it in the yard and I have done a fair bit since, mostly on trawlers.
Thanks for the info on the Tercoo. I watched some of the promotional videos for it and wasn't much impressed but it does look as though it could be useful for stripping back soft or gummy coatings which are slow to blast because they absorb grit.

Yes, the range of movement is small so the tacks don't usually crack when pushing on the lever but on the other hand when you're done you can easily twist it free by hand. If the tacks to the lever do crack then maybe try them with 7018 rods which is more crack resistant. Obviously, you'll have to grind all the tacks off after so you want them to be as small as possible.

When you perimeter weld after, keep down the length of continuous weld and keep the welds as small as you can consistent with doing the job. The long edge welds will cumulatively contract which has an effect similar to pulling a thread in a loose weave fabric causing the fabric to ruffle either side of the thread. You can see lots of chine steel hulls with a wave in the chines that is caused by weld shrinkage. I'm guessing that there probably won't be much framing on the inside of your hull to keep the plates fair.

Good luck with that...it's depressing; I well know! If you are careful though you should be able to recycle most of the material when putting it back.

Post some photos of progress..